1. Field of the Invention
The present invention relates to a strained multiple quantum well semiconductor laser which can be used as a light source for optical communication or optical disks, and to a method for producing the same.
2. Description of the Related Art
In a quantum well structure, it is known that a well layer formed to a critical thickness or less can be made to have compressive strain or tensile strain inside by prescribing the lattice constant of the well layer to be larger (or smaller) than that of a substrate (N.B. a critical thickness: a thickness at which dislocations occur within crystals so as to release lattice mismatching). This technique, which allows one to flexibly design the energy band structure of the quantum well by introducing strain to the inside of the well layer, has been earnestly studied in recent years. In particular, a semiconductor laser in which such a strained quantum well structure is used as an active layer is capable of emitting light in a wavelength band that a lattice-matched type laser cannot emit. Moreover, such a semiconductor laser is expected to show improved performance in emission of light in wavelength bands that a lattice-matched type laser does emit.
Hereinafter, a semiconductor laser having such a strained quantum well structure in a long-wavelength band of 1.55 .mu.m, which is used as a light source for optical communication, will be described.
Light An a wavelength band of 1.55 .mu.m is known to suffer the smallest loss, among light in other wavelength bands, when transmitted through an optical fiber. An exemplary semiconductor laser of this wavelength band of 1.55 .mu.m, whose active layer has a quantum well structure including an InGaAs well layer and an InGaAsP barrier layer, will be described.
In order to introduce compressive strain to the InGaAs well layer, it is necessary to make the mole fraction of Ga smaller than 0.47. Conversely, in order to introduce tensile strain to the InGaAs well layer, it is necessary to make the mole fraction of Ga larger than 0.47. In particular, by introducing compressive strain to the well layer, the state density of heavy holes disposed on top of the valence band is lowered. As a result, the carrier density required for degeneration is lowered. Therefore, Bernard-Duraffourg's requirements for laser oscillation can be satisfied at a low carrier density. Therefore, recombination of spontaneous emission is restrained, thereby lowering the threshold current. It is pointed out that, while a semiconductor laser in a long-wavelength band, especially that of 1.55 pm, has a large Auger recombination component in its threshold current, introduction of compressive strain to the well layers thereof lowers its threshold carrier density, thus drastically reducing the Auger recombination current. Due to the lowered carrier density, the half-hand width of the gain spectra is reduced, Thereby increasing the differential gain of the semiconductor laser.
Such a semiconductor laser, whose active layer has a quantum well structure in which compressive strain is introduced to the well layers, has already been realized, and is reported to have a lower threshold current density, an improved characteristic temperature, and a higher differential gain. FIG. 5 illustrates the structure of a conventional example of such a semiconductor laser. The semiconductor laser includes an n-InP substrate 501, an InGaAsP waveguide layer (A) 502 having an appropriate composition for obtaining light in a wavelength band of 1.3 .mu.m, an active layer 503 having a strained multiple quantum well structure, In.sub.0.7 Ga.sub.0.3 As well layers 504, InCaAsP barrier layers 505 having an appropriate composition for obtaining light in a wavelength band of 1.3 .mu.m, an InGaAsP waveguide layer (B) 506 having an appropriate composition for obtaining light in a wavelength band of 1.3 .mu.m, and a p-InP cladding layer 507. Compressive strain of about 1.2% is introduced to the In.sub.0.7 Ga.sub.0.3 As well layers 504. Four In.sub.0.7 Ga.sub.0.3 AS well layers 504 are provided. The thickness of each In.sub.0.7 Ga.sub.0.3 AS well layer 504 is 4 nm. The thickness of each InGaAsP barrier layer 505 is 10 nm.
In optical communication, transmission of a large amount of information requires a semiconductor laser capable of operating at a very high speed. However, the upper limit of the response speed of a semiconductor laser is dependent on the relaxation frequency (fr) inherent to the semiconductor laser. The relaxation frequency (fr) is derived from the following Equation (1): ##EQU1## where A, r, V.sub.a, q, I, and I.sub.th respectively represent the differential gain, the light-confinement coefficient with respect to all the well layers, the total volume of the well layers, electron charge, the amount of the injected current, the amount of the threshold current. If strain is introduced to the well layers, the differential gain can be made about twice as large as that of a semiconductor laser having a non-strained quantum well structure. In a case where strain is introduced to well layers made of InGaAs, however, varying the composition thereof, as is necessitated for generating the strain, inevitably causes the energy band gap as well as the lattice constant to vary. For example, in a case where compressive strain is to be introduced, the lattice constant of InGaAs in a bulk state should be made larger than that of InP, of which the substrate is to be made. Accordingly, the In content of each InGaAs well layer should be increased. In this case, the energy band gap of the InGaAs well layer shifts toward the low-energy side. As a result, the oscillation wavelength becomes larger than 1.55 .mu.m, provided that the thickness of each InGaAs well layer is made the same as that of an InGaAs well layer of a semiconductor laser having a non-strained quantum well structure (i.e. 6 nm), but such a semiconductor laser is not practical in optical communication. In order that the oscillation wavelength be 1.55 .mu.m, each InGaAs well layer should be made as thin as 2 to 4 nm, thereby obtaining an quantum size effect so that the oscillation wavelength shifts to the long-wavelength side. However, employing a thinner well layer inevitably lowers the light-confinement coefficient, therefore lowering the value .GAMMA./V.sub.a. FIG. 6 illustrates the dependence of the value .GAMMA./V.sub.a on the number of well layers, with respect to a case where the thickness of each well layer is 6 nm and a case where the thickness of each well layer is 3 nm. As is seen from FIG. 6, when seven well layers are provided, reducing the thickness of each well layer from 6 nm to 3 nm lowers the value .GAMMA./V.sub.a by 20%. As has been described, it is necessary to provide a sufficiently large number of well layers so that the increase of differential gain due to the introduction of strain to the well layers will effectively raise the relaxation frequency.
In a multiple quantum well structure, introducing strain to the well layers thereof also introduces strain to the barrier layers thereof, according to the principle of action and reaction. FIGS. 7A and 7B each illustrate how the amount of strain varies along the direction that the layers are laminated. FIG. 7A describes a case where the strained well layers are made of InGaAs and the barrier layers are made of InP, while FIG. 7B describes a case where the strained well layers are made of InGaAs and the barrier layers are made of InGaAsP. As is seen from FIG. 7A, deformation of lattice occurs due to the tensile stress 703 introduced by well layers 702, to which compressive strain is introduced. However, it takes only several atomic layers for the lattice constant to be brought back to that of a non-strained state, since the barrier layers 701 consist of a binary material of InP. Therefore, tensile strain 704 due to lattice deformation exists in only the interface between each barrier layer 701 and each well layer 702. In other words, each barrier layer VOL is fairly well brought back to a non-strained state before the next well layer 702 is formed, on conditions that each barrier layer 701 has a thickness of only a few nanometers. Thus, it becomes possible to grow as many strained well layers 702 as desired.
As is seen from FIG. 7B, on the other hand, deformation of lattice occurs due to the tensile stress 703 introduced by well layers 702, to which compressive strain is introduced. In this case, however, since barrier layers 705 are made of a quaternary material, InGaAsP, the arrangement of atoms changes, and therefore the barrier layers 705 tend to grow with the tensile strain stored inside. A thickness 706 of each barrier layer 705, which is about 10 nm, is not sufficiently large to bring back the lattice constant to a non-strained state.
In fabrication of such a semiconductor laser, the tensile strain accumulates within the barrier layers 705 as the well layers 702 and the barrier layers 705 keep being alternatively grown upon one another, until the tensile strain reaches a critical level 708. Past the critical level 708, dislocations due to lattice relaxation occur within the barrier layers 705. These dislocations eventually spread among the whole quantum well structure, making it impossible to obtain a quality quantum well structure. For example, in a case where compressive strain of about 1% is introduced to each InGaAs well layer 702 having a thickness of 10 nm, the light emitting performance of the semiconductor laser drastically drops when 10 InGaAs well layers 702 have been grown. This is presumably because the tensile strain stored within the barrier layers 705 reaches the critical level when 10 InGaAs well layers 702 have been grown. On the other hand, compressive strain of at least 1% must definitely be introduced to the well layers 702 so as to attain a substantial improvement in the light emitting performance. Therefore, in a conventional strained multiple quantum well semiconductor laser, only 10 or less well layers can be formed when compressive strain of 1% is introduced to the well layers. Accordingly, such a strained multiple quantum well semiconductor laser is expected to show little enhancement in the relaxation frequency thereof, as compared with a non-strained multiple quantum well semiconductor laser having more than 10 well layers formed.